This invention relates to certain new and useful improvements in two-wheeled hand trucks or dollies of the type wherein a primary frame mounted on a pair of wheels is adapted in use to assume a generally vertical position, and has a generally forwardly disposed nose adapted to transport loads which are of such bulk and weight that it is not practical to attempt to carry them without assistance. More particularly, the invention relates to hand trucks of the type disclosed in U.S. Pat. No. 4,762,333 entitled "Repositionable Hand Truck", wherein wheel-supporting brackets are pivotably connected to the frame in a manner to permit movement of the wheels from a first load supporting position, in which the wheels are adjacent to the lower end of the frame, to a second load supporting position in which the wheels and axle are moved materially rearwardly for supporting loads with a relatively higher center of gravity.
In this patent, available energy storing and pressure exerting mechanism, in the form of gas cylinders, operate through moment arms on opposite sides of the bracket pivots to releaseably retain the wheel structure in each of its positions. While fixed-position stair climbers have been employed on more conventional hand trucks in a position in which their linear rear surfaces are in lateral alignment with the rear peripheral surfaces of the wheels of the hand truck, no one has, to my knowledge, developed a stair climber structure for hand trucks of the type disclosed in the patent which automatically is shifted to such a position of alignment when the load supporting position of the wheels is shifted to suit the particular load to be carried in the manner disclosed in the aforesaid patent.
In the present assignee's U.S. Pat. No. 3,997,182, the stair climber members employed have selectable alternate openings enabling attachment to the fixed brackets of the hand truck at a differing location when it is desired to change the wheels on the hand truck to wheels of differing size. Such stair climber rails need to be uncoupled, and then recoupled, using the alternate set of openings, a clip securing the upper ends of the stair climbers to the side rails also needs to be unfastened so that the stair climbers can be slid upwardly or downwardly with respect to the side rails of the hand truck, to the extent desired. None of this occurs automatically, and could not, because, the brackets and axle are fixed. Moreover, such selectable openings cannot be used with the hand truck disclosed in U.S. Pat. No. 4,762,333.